1. Field of the Invention
The invention relates generally to the alkylation of aromatic compounds and catalysts used for such reactions and their preparation.
2. Description of the Prior Art
Before 1940 virtually all of the aromatic solvents, including xylene, were produced from coal. Thereafter production of xylene from petroleum started. Most mixed xylene is currently produced by catalytic reforming of petroleum aromatic-rich streams from refineries. It is also obtained from pyrolysis gasoline as a by-product of olefin manufacture during the cracking of hydrocarbons, by-product of naphtha cracking. Xylene can also be obtained from toluene by disproportionation or alkylation. Toluene disproportionation (TDP) is a catalytic reaction of toluene to produce xylenes and benzene. Toluene methylation (TM) is a catalytic reaction of toluene with methanol to produce xylenes as shown below:

All of these processes generally produce a mixture of isomers. The xylene isomers, meta-xylene (m-xylene), ortho-xylene (o-xylene) and para-xylene (p-xylene), are important chemical intermediates. o-Xylene is oxidized to make phthalic anhydride which is used to make phthalate plasticizers among other things. m-Xylene is oxidized to make isophthalic acid, which is used in unsaturated polyester resins (UPR). However, p-xylene has by far the largest market of the three isomers. The largest use of p-xylene is in its oxidation to make terephthalic acid. Terephthalic acid is used in turn to make polymers such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT). PET is one of the largest volume polymers in the world. As such the demand for p-xylene is several times that for m- and o-xylene. In commercial manufacture p-xylene is purified from mixed xylenes by crystallization and adsorption processes.
Thermodynamic equilibrium compositions of o-, m-, and p-xylenes are approximately 25, 50 and 25%, respectively, at 500° C. The catalytic processes such as TDP and TM would give about 25% p-xylene (PX) in mixed-xylenes (MX). However, if a catalyst possesses shape selective properties it will give significantly greater than 25% PX. Typically, a shape selective catalyst would give >85% PX in MX.
Zeolites are crystalline solids made up of aluminum-substituted SiO4 tetrahedral units joined together to form different ring and cage structures into a crystalline framework. The physical structure of zeolite is very porous with a large internal and external surface area. The substitution of aluminum generates a charge imbalance which must be countered by a supplementary counterion, such as a proton.
Zeolites can be shape-selective catalysts due to steric and electronic effects. Selective reactions can occur over zeolites as certain products, reactants or transition states are kept from forming within the pores either by transition state selectivity or because of size or shape of molecular diameter. By varying the preparation of zeolite catalysts, they can be modified to carry out very specific syntheses of desired products.
Modified zeolite catalysts are known for alkylation of aromatics, specifically methylation of toluene to xylenes, especially p-xylene

U.S. Pat. No. 6,504,072 discloses a phosphorus-modified zeolite used in the selective methylation of toluene to p-xylene. The P-modified zeolite catalyst may contain active and inactive materials such as clays, silica and/or metal oxides such as alumina as a binder. There is no disclosure of modification or treatment of the binder and no disclosure of the effect of modification or treatment of the binder on selectivity to para-xylene.
Other prior art discloses the modification or treatment of binder material for zeolite catalysts used in other processes.
U.S. Pat. No. 5,907,073 discloses a process for alkylation of an aromatic with a molecular sieve catalyst of a modified zeolite beta having an intergrowth of a ZSM-12 crystalline framework. During synthesis, NH4-Beta and LaNH4-Beta were mulled with nitric acid treated alumina, extruded and calcined which converted the NH4-Beta and LaNH4-Beta into H-Beta catalyst and LaH-Beta catalyst, respectively. The finished catalyst was used in ethylation of benzene to form ethylbenzene. There is no disclosure that modification or treatment of the binder had an effect on selectivity.
U.S. Pat. No. 5,380,690 discloses a catalyst for the production of light olefins which is a mixture of clay, inorganic oxides and zeolite prepared by mixing the precursor of the inorganic oxides such as aluminum sol, pseudo-bohemite, silica sol or its mixture, and silica-alumina sol or gel with clay, peptizing with de-cationized water to prepare a slurry, further mixing homogeneously, adjusting and maintaining the pH value of the slurry to 2-4 using inorganic acid such as hydrochloric acid, nitric acid, phosphoric acid, or sulfuric acid, after aging statically, adding into it a pre-calculated amount of zeolite, homogenizing, spray drying, washing-off isolated sodium ions, and drying. There is no disclosure that adjusting and maintaining the pH of the slurry containing the binder precursor had an effect on selectivity.
U.S. Pat. No. 6,047,544 discloses an engine exhaust gas purification catalyst of a layer of palladium, platinum or rhodium on a layer of a zeolite hydrocarbon adsorbent on a monolithic substrate. During synthesis, Pd-impregnated alumina powder, Pd-impregnated cerium oxide powder, nitric acid acidified alumina sol and water were introduced into a magnetic ball mill to crush the mixture so as to obtain a slurry. The nitric acid acidified alumina sol was obtained by adding 10 weight % nitric acid to 10 weight % bemite alumina. There is no disclosure that modification or treatment of the alumina had an effect on selectivity.
U.S. Pat. No. 6,399,530 discloses a hydrocracking catalyst having a binder of a small pore alumina which is peptized with nitric acid and has a specific surface area of 240-280 m2/g and a pore volume of 0.4-0.5 ml/g. There is no disclosure that modification or treatment of the alumina binder had an effect on selectivity.
U.S. Pat. No. 6,566,293 discloses an olefin-selective catalyst for production of light olefins which is a phosphorus-modified zeolite having a binder. The alumina binder is usually peptized before or during its combination with the other catalyst composition components. There is no disclosure that peptization of the alumina binder was with a mineral acid or that peptization of the alumina binder had an effect on selectivity.